The signature hypothesis: co-localizations of neuroactive substances as anatomical probes for circuitry analyses.

The recent discoveries that a neuron in the vertebrate retina may contain more than one neuroactive substance (transmitter or neuropeptide) raise the possibility that within each class of neurons, every morphologically and physiologically distinct cell type may be uniquely identified and categorized by the neuroactive substances that it contains. This article examines the evidence to-date for such a conjecture and discusses some of its potential applications and implications.